In recent years considerable effort has understandably been directed toward the utilization of solar energy. While numerous advances have been made in the art, many systems are more theoretical than practical, and the problem of providing a low cost and yet reasonably efficient heat transfer panel remains.
An economical and yet efficient heat transfer panel could advantageously be used not only to collect solar energy, but also to radiate the collected energy to heat a structure. Conversely, a heat transfer panel can be used to absorb unwanted heat from a structure, with a second similary formed panel used to radiate the absorbed heat to a black body, such as the night sky.
The heat transfer panel of the present invention relates to panels which are generally known as "flat plate collectors," although as above noted they can be used to reject heat as well as collect the same. Typical of prior art flat plate collectors are the devices shown in U.S. Pat. Nos. 3,146,774, 3,918,430, 3,943,911, 3,965,887 and 3,989,031. In the first of these patents, U.S. Pat. No. 3,146,774, water is distributed over an inclined planar surface and under a thin film held at a spaced distance from the surface by guide rods. Theoretically, as long as the flow rate is relatively low, the surface tension of the water flowing between the film and the backing of surface will cause the film to be drawn down toward the backing surface to relatively evenly distribute water over the backing surface for absorption of heat. Unfortunately, the distribution of water may be, and usually is, distributed over the planar support surface in less than an optimal manner. If the panel is not transversely level (a condition likely to occur on most roofs), water will gravitate preferentially toward the low side. Similarly, sagging of the support surface will cause preferential flow. The remaining four referenced patents are directed to flat plate collectors having either channels or protruberances which channel or divert flow of trickling water over a plate, usually with a blackened surface to absorb radiant heat. These panels further include additional glazing sheets to enhance heat transfer to the fluid by reducing convection and re-radiation losses from the collector. In such systems, however, condensation of water on the fluid channel defining surfaces or the glazing surfaces can be a problem, as can be the cost of construction of the panels.
U.S. Pat. Nos. 3,077,190 and 3,620,206 disclose solar energy collection systems which are designed for use in a horizontal orientation. Thus, these apparatus are positioned on substantially horizontal surfaces, but this horizontal orientation causes these systems to have a much lesser efficiency than can be achieved by an inclined, flat plate collector. It is known, for example, that for winter heat collection from solar radiation a collector panel should optimally be oriented at an angle from the horizon equal to the latitude of the location of the collector plus about 20.degree. for maximum absorption of radiant energy from the sun. At a location having a 40.degree. north latitude, the collector should be oriented at 60.degree. from the horizon in a south or southwest facing direction. In addition to the horizontal systems of the above set forth patents, it is also known to position glazing materials on top of ponds to enable the ponds to act as solar collectors. Such glazing materials have taken a number of different configurations including the floating of sheets and tubes of transparent material on the ponds. Such glazing creates a "greenhouse" effect by reducing convection and re-radiation from the pond. Such horizontally oriented systems, however, will always suffer from the defect of being oriented at an angle to the sun which is relatively inefficient for the collection of solar energy. Accordingly, they require substantial area or land for any given quantity of heat absorption.
In U.S. Pat. No. 3,991,742, a solar energy collector is disclosed which is comprised of a large bag or panel formed from flexible sheets of material, which panel is positioned on a sloped surface such as a roof. The panel is preferably formed in a manner which enhances distribution of the flow of water between the sheets comprising the panel. Roof sagging and lateral slope, however, will cause preferential flow. Moreover, such an approach encounters heat transfer problems in that the upper surface will not always be in contact with the moving water, particularly because starting of the pump will invariably introduce air into the bag.
Finally, an article entitled "Solar" in the November, 1976 issue of Sunset Magazine, pages 79-89, discloses several types of flat plate solar collectors. Collectors are described which employ tubes and heat absorption plates, rigid sandwiches of metals, and open, corrugated panels having water trickled over upwardly facing surfaces. These systems, however, make various unsatisfactory tradeoffs between cost and efficiency. In closed, pressurized systems leakage is a constant problem. In addition to the problem of the cost of copper tubing, the problem of galvanic action is often present between the copper and aluminum absorption plates. Galvanic action inhibitors interface with heat transfer and therefore cut down efficiency. In open systems costing less, evaporation of the collector fluid is particularly troublesome, since it will reduce fluid temperature and collect on glazing surfaces to substantially further reduce heat transfer efficiency.